Chemical Bonding What are two elements joined together

Chemical Bonding

What are two elements joined together called? • Two elements chemically bonded together are called a compound. • A compound has different properties than the elements that make it up • A compound is represented by a chemical formula

Chemical Formulas • A chemical formula tells the elements that make up a compound and the number of atoms of each element in one unit of it. • A compound always has the same chemical formula H 2 0 Na. Cl Si 02 C 12 H 22 O 11

Why do elements bond? • Elements bond together in order to achieve stability • The noble gases rarely bond because they are already stable • An atom is chemically stable when it’s outer energy level is complete • Atoms do this by gaining, losing, or sharing electrons with other atoms • An atom that has lost or gained an electron is called an ion

Valence Electrons • Valence electrons are the electrons in the highest occupied energy level of an elements atoms. • Valence electrons can be found by looking at the periodic table.

Ion Formation - Cation • An ion with a positive charge is called a cation and is formed when an atom loses electrons • This happens most commonly with metals • An atom can become more stable by ionizing • Na: 1 s 22 p 63 s 1 -> Na+: 1 s 22 p 6 octet • Sodium now has the same electron configuration as neon by losing one electron

Ion Formation - Anion • An anion is formed when an atom gains electrons • This happens most typically with nonmetals • Cl: 1 s 22 p 63 s 23 p 5 -> Cl-: 1 s 22 p 63 s 23 p 6 octet • Ions produced from the atoms of the halogen group are called halide ions

Types of Bonding - Ionic • Compounds formed from cations and anions are called ionic compounds • The net charge on the compound is zero • The electrostatic forces that hold the compound together are called ionic bonds • An ionic bond is usually found between metals and nonmetals

Ionic Compounds • Ionic compounds exist as a collection of positivly and negativly charged ions arranged in a repeating pattern. • The formula for ionic compounds is refered to as a formula unit, or the lowest whole-number ratio of ions in an ionic compound

Properties of Ionic Compounds • Ionic compounds are usually crystal solids at room temperature • The components are arranged in repeating three dimensional patterns • In solid Na. Cl, each sodium ion is surrounded by 6 chloride ions and each chloride ion is surrounded by 6 sodium ions • Each ion is strongly attracted to it’s neighbor and repulsion effects are minimized • The large attractive forces result in a stable structure with a high melting point

Properties of Ionic Compounds • The coordination number of an ion is the number of all ions of opposite charge adjacent to it in a crystal • Na. Cl has a coordination number of 6 • If the chemical formula of an ionic compound does not contain equal parts of each element, each element will have a different coordination number • In Ti. O 2, Ti 4+ has a coordination number of 6 while O 2 - has a coordination number of 3

Properties of Ionic Compounds • Ionic compounds can conduct an electric current when dissolved or melted in water • When the substance is dissolved in water, ions are free to move from one electrode to another which produces a flow of electricity

Metallic Bonding • A metallic bond is a bond between two metals • The valence electrons of metal atoms can be viewed as a sea of electrons, they are mobile and can drift freely from one part of the metal to another

Metallic Bonding • This sea of electrons explains many of the physical properties of metals • Metals are good conductors of electricity because electrons can flow freely in them, as electrons enter one end, an equal number leave the other • Metals are ductile and malleable, meaning they can be stretched into wires and hammered into sheets • This is because the sea of valence electrons insulates the metal cations from one another. When subjected to stress, the metal cations slide past each other like ball bearings in oil. • When ionic compounds are subjected to force, like charges come into contact and the crystal shatters

Crystalline Structure of Metals • Like ionic compounds, metals also have a crystalline structure • Metal atoms are arranged in very compact and orderly patterns and are the simplest forms of crystalline structure • For spheres of equal size there are many arrangements possible, three of which are body centered cubic, face centered cubic, and hexagonal close packed

Body Centered Cubic • In a body centerd cubic arrangement, every atom except those on the surface has eight neighboring atoms • Sodium, potassium, iron, chromium, and tungsten crystallize into this form

Face Centerd Cubic • In a face centerd cubic arrangement, every atom except those on the surface has twelve neighboring atoms • Copper, silver, gold, aluminum, and lead crystallize in this form

Hexagonal Close Packed • In a hexagonal close packed arrangement, every atom except those on the surface has twelve neighboring atoms, but in a different arrangement • Magnesium, zinc, and calcium crystallize into this form

Alloy • An alloy is a mixture of a metal and another element. • The second element makes the mixture stronger than it’s main element. • The second element enhances the properties of the main element • Two of the most well known alloys are steel and brass

Alloys • If the atoms of the second element are of a similar size to the main element, they can take the place of the main element in the crystal structure, and a substitutional alloy is formed • If the atomsof the second element are small enough to fit into the interces of the main element, a intersitial alloy is formed • Steel is an intersitial alloy

Molecules and Covalent Compounds

Molecules and Covalent Compounds • In nature, only noble gases exist as atoms. • Atoms can form bonds that are not due to electrostatic forces. • They can share electrons in a covalent bond. • Many elements in nature are found in the form of molecules. • A molecule is an electrically neutral group of atoms joined together by covalent bonds

Molecular Compounds • A compound composed of molecules is a molecular compound • A molecule consisting of two of the same atom is called a diatomic molecule. Example Oxygen • Molecular compounds tend to have relatively low melting and boiling points • Molecular compounds are usually formed from two or more nonmetals

Molecular Formulas • The chemical formula for a molecular compound is called a molecular formula, and shows the kinds and numbers of atoms present in a compound • It does not tell you about the structure of the molecule itself NH 3

Types of Bonding - Covalent • Losing electrons takes energy, and if the energy required is to much, atoms will share electrons • A single covalent bond is made up of two shared electrons, usually one from each atom in the bond • This bond results in a stable outer level for each atom or noble gas configuration

Single Covalent Bonds • Two atoms held together by sharing a pair of electrons are joined by a single covalent bond • When writing the electron dot diagram of molecules, the electrons that are shared can either be shown as two dots, or a dash H: H or H-H

Multiple Covalent Bonds • A covalent bond can contain more than one pair of electrons • An example is N 2 , which shares 3 pairs of electrons between the two nitrogen molecules which is a triple bond

Coordinate Covalent Bonds • A coordinate covalent bond is a bond in which one atom contributes both bonding electrons. • An example is carbon monoxide. Carbon needs 4 electrons to become stable, and oxygen needs two. For the final two carbon needs, oxygen donates them. This is represented by drawing an arrow from oxygen to carbon.

Polyatomic Ions • A polyatomic ion is a group of atoms tightly bound together with a positive or negative charge that function as a unit • They are held together with covalent bonds and coordinate covalent bonds • Polyatomic ions can then form ionic bonds with other atoms

Bonding • The total energy required to break the bond between two covalently bonded atoms is known as the bond dissociation energy. • A large bond dissociation energy corresponds to a strong covalent bond

Polar & Nonpolar Molecules • Electrons are not always equally shared between atoms. • More electronegative atoms attract electrons more • Because of this, one end gains a slight positive charge, and the other a slight negative charge • This is called a polar covalent bond • Molecules made of identical elements or are symmetric are nonpolar molecules

Attractions between molecules • Molecules can attract each other in a variety of ways. • These methods are weaker than bonding forces but still can affect molecules • The two main ones are called Van der Waals Forces and hydrogen bonding. • The two weakest attractions are called Van der Waals Forces • Dipole interactions occur when opposite ends of polar molecules attract eact other • Dispersion forces occur between nonpolar molecules and are the weakest of all interactions • They occur when electrons momentarily move to one side of an atom causing the electrons in the neighboring atom to move momentarily making it polar

Hydrogen bonding • A hydrogen bond occurs when hydrogen is bonded to an extremely electronegative element and is also very weakly bonded to an unshared pair in another electronegative atom. The resulting bond has about 5% the strength of a covalent bond.

Lewis Structures

Lewis Structures • A Lewis Structure is a way of representing a molecule and the bonds that it shares. • A pair of electrons that is shared between two atoms is a shared pair and is represented by a dash (-) • An unshared pair of electrons is called a lone pair or nonbonding pair and is represented by two dots

Resonance Structures • A resonance structure is a structure that occurs when it is possible to write two or more electron dot diagrams for the same molecule or ion

Lewis Structures 1. First figure out the total number of electrons available for bonding by writing the electron dot diagram for each element 2. Next figure the total number of atoms to get the noble gas configuration for each atom. 3. Subtract the first number from the second to get the total number of bonding electrons. 4. Divide that by 2, that is the number of bonds in the Lewis structure 5. When writing the bonds, hydrogen and halogens bond once, oxygen group bonds twice, nitrogen group 3 times, and carbon group 4. 6. To get the number of unbonded electrons, subtract 3 from 1 7. Pick the central atom, and write it first 8. Write in the bonds to give noble gas configuration 9. Write in any unshared electron pairs.

Bonding Theories • Molecules are not flat as a Lewis structure would make them appear, they are actually three dimensional • VSEPR Theory explains the three dimensional shape of molecules. • VSEPR stands for valence shell electron pair repulsion • It states that the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible • Unshared pairs repel more strongly than shared pairs because they are held closer to the nucleus of the atom than bonded pairs